Bone remodeling is the dynamic process by which tissue mass and skeletal architecture are maintained. The process is a balance between bone resorption and bone formation, with two cell types, the osteoclast and osteoblast, thought to be the major players. Osteoblasts synthesize and deposit new bone into cavities that are excavated by osteoclasts. The activities of osteoblasts and osteoclasts are regulated by many factors, systemic and local, including growth factors.
Many of the proteins that influence the proliferation, differentiation, and activity of osteoblasts, osteoclasts, and their precursors also affect these processes in chondrocytes, the cells responsible for cartilage formation (chondrogenesis). These proteins include platelet-derived growth factor (PDGF), insulin-like growth factor (IGF), basic fibroblast growth factor (bFGF), transforming growth factor beta (TGF-β), bone morphogenetic proteins (BMP), and cartilage-derived growth factor (CDGF).
The exact mode by which PDGF affects the growth of osteoblasts is not yet clearly understood, however, this growth factor is generally believed to play a key role in the regulation of both normal skeletal remodeling and fracture repair. Biologically active PDGF is found as a homodimer or a heterodimer of the component A and B chains. In vitro studies have shown PDGF to be mitogenic for osteoblasts (Abdennagy et al., Cell Biol. Internat. Rep. 16(3):235–247, 1992). Mitogenic activity as well as chemotactic activities associated with PDGF have been demonstrated when the growth factor is added to normal osteoblast-like cells (Tuskamota et al. Biochem. Biophys. Res. Comm., 175(3):745–747, 1991) and primary osteoblast cultures (Centrella et al. Endocrinol. 125 (1):13–19, 1989). Recent studies have demonstrated that the osteoblast produces the AA isoform of PDGF (Zhang et al., Am. J. Physiol. 261:c348–c354, 1991).
PDGF has been shown to be useful for promoting the repair of both soft and hard tissues. For example, PDGF has been shown to promote the regeneration of bone and ligament in patients suffering from periodontal disease (Howell et al., J. Periodontol. 68:1186–1193, 1997). As disclosed in U.S. Pat. No. 5,533,836, PDGF stimulates the growth of osteoblasts, and this activity is enhanced in the presence of vitamin D. PDGF has also been shown to promote the healing of gastrointestinal ulcers (U.S. Pat. No. 5,234,908) and dermal ulcers (Robson et al., Lancet 339:23–25, 1992; Steed et al., J. Vasc. Surg. 21:71–81, 1995). The use of PDGF for stimulating chondrocyte proliferation and regenerating cartilage is disclosed in U.S. Pat. No. 6,001,352.
A PDGF homolog known as “zvegf3” was recently identified (U.S. patent application Ser. No. 09/457,066). This protein has also been designated “VEGF-R” (WIPO Publication WO 99/37671). A related protein, designated “zvegf4”, has also been identified (U.S. patent application Ser. Nos. 09/304,216 and 60/132,250). Zvegf3 and zvegf4 are multi-domain proteins with significant homology to the PDGF/VEGF family of growth factors. WO 99/37671 discloses that VEGF-R is an angiogenic factor.
Despite the increasing knowledge of the role of growth factors in tissue growth and repair, there remains a need in the art for materials and methods for promoting the growth of bone, ligament, and cartilage. There also remains a need the art for materials and methods for modulating the proliferation and differentiation of cells in vitro and in vivo.